Group theory basically applies to any set with more than one person. A 2 set of two girls requires group theory. A mixed 2 set requires the PUA to quickly identify their relationship based on body language and conversation. 3 sets, 4 sets, 5 sets, and bigger groups all have different group dynamics.
Different methods and tactics have been developed for the various possible group combinations in the community. In general:
Group theory comes into play in all social gathering situations.